Today, more and more output systems are developed for the reproduction of color images. Several display and printing technologies are used such as CRT's, LCD's, conventional photography, electrophotography, thermal transfer, dye sublimation and inkjet systems to name a few. In the rest of this document, these systems will be referred to as output devices.
All these systems can be described as multi-dimensional output devices with n colorants such as CMYK (cyan, magenta, yellow and black) inks of an inkjet system or RGB (Red, Green, Blue) in case of a display system. In this document it is assumed that the colorant values for printers range from 0% (no colorant laid down on paper) to 100% (maximum amount of colorant laid down on paper). For display systems, the values range from 0 to 255. In the rest of this document, mainly a printer will be used as an example of an output system, however, it is well known in the art of color management systems that all aspects of printers can be easily extended to those of a display systems.
With colorant space is meant an n-dimensional space with n the number of independent variables with which the output device can be addressed. In the case of an offset printing press the dimension of the colorant space corresponds to the number of inks of the printer. As normally CMYK inks are used, the dimension of the colorant space is four. Colorant spaces are also referred to as device dependent spaces.
The colorant gamut is defined by all possible combinations of colorant values, ranging from 0% to 100% for printers and from 0 to 255 for display systems. If there are no colorant limitations, the colorant gamut is a n-dimensional cube.
With color space is meant a space that represents a number of quantities of an object that characterize its color. In most practical situations, colors will be represented in a 3-dimensional space such as the CIE XYZ space. However, also other characteristics can be used such as multi-spectral values based on filters that are not necessarily based on a linear transformation of the color matching functions. The values represented in a color space are referred to as color values. Color spaces are also referred to as device independent spaces.
A printer model is a mathematical relation that expresses color values in function of colorants for a given output system. The variables for the colorants are denoted as c1, c2, . . . , cn with n the dimension of the colorant space. An n-ink process is completely characterized by its colorant gamut with a number of colorant limitations and the printer model. Because of this close relationship between an n-ink process and the printer model, the operations typically defined for a printer model are easily extended to an n-ink process.
The printer model is often based on a printer target. Such a target consists of a number of uniform color patches, defined in the colorant space of the printing device. In a next step the printer target is printed and measured, and based on the values of the patches in colorant space and the measured color values, the printer model is made. A printer target is normally characterized by the sampling points along the different colorant axes. Based on the sampling points a regular grid can be constructed in colorant space of which a number of grid points are contained by the printer target. Hence a target can be said to be complete or incomplete. (see EP-A-1 146 726, herein incorporated by reference in its entirety for background information only, for regular grids and for complete and incomplete printer targets).
With inverting an n-ink process is meant that the corresponding printer model is inverted. In this way, a so-called characterization transformation is obtained, that transforms colors from color space to the colorant space of the concerned printer. For more information on characterization, calibration and other relevant terms in color management, we refer to patent application EP-A-1 083 739, incorporated herein in its entirety for background information only. As opposed to the characterization transformation, the transformation of an n-ink process to color space is equivalent to the transformation of the corresponding colorant domain to color space by making use of the printer model.
In graphic arts, it is common to simulate a job to be printed on an output device. This simulation process is called proofing and the print is referred to as the proof.
There may be several reasons to make a proof, such as:                if prints have to be made in a rather large number of copies, in most cases printing systems are selected that result in a low cost per copy. However, the disadvantage of most of these systems is that the setup costs are rather high. To check the setup, a proof can be created based on the workflow just before the printing system;        to check the design;        to check the layout of a page, e.g. to check if all page elements are present;        to check moiré effects; and        to check the color.        
As there are many reasons to make a proof, the required quality of the proof may depend on the circumstances; e.g. if the layout of a proof has to be checked, the color accuracy is less important.
Patent application US 2002/0008880 A1, herein incorporated by reference in its entirety for background information only, discloses a color proofing method and apparatus.
There is still a need for an improved method for making a proof.